[Haskell-cafe] cabal sdist warns about optimization levels

Daniel Fischer daniel.is.fischer at googlemail.com
Sun Jan 13 23:00:04 CET 2013

On Sunday 13 January 2013, 21:27:44, Petr P wrote:
> I wonder:
> (1) Is there a way how to disable the warning? As the main aim of the
> library is speed, I believe -O2 is appropriate here. And since the code is
> quite short, I'm quite sure the increased compile time won't be noticeable.
> (2) Why does cabal complain about it at the first place? I found a
> reference saying the warning is adequate:
> https://github.com/haskell/cabal/issues/808
> but not saying why. Maybe for complex programs -O2 prolongs compile time
> too much, but libraries are usually compiled once and used many times, so
> using -O2 for them seems reasonable in many cases.

Sometimes compiling with -O2 instead of just -O takes significantly longer.
Not always is the produced result faster (often, the results are identical).

So if the code produced with -O performs equally to that produced with -O2, 
and the -O2 compilation takes significantly longer, choosing -O2 imposes a 
cost for no benefit.

That's, I think, why the warning is considered adequate.

You can specify -O2 on a per-module basis with an
{-# OPTIONS_GHC -O2 #-}
pragma where it matters, then cabal won't complain.

Or, if you're too lazy to check the consequences of -O2 vs. -O for each module 
(like I usually am, if there are more than a handful), just verify that -O2 
does indeed make a significant difference for the speed of the result in some 
places without increasing compile time unduly, and henceforth ignore the 
warning if it does. (Re-test every couple of compiler versions.)
After some time, you tend to not even notice it anymore ;)


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